1. Field of the Invention
The present invention relates to a method of suppressing false color.
2. Related Background Art
A conventional digital camera adopting a single plate system has such a structure as shown in FIG. 10. In FIG. 10, light 1 reflected from a subject passes through a photographing lens 2, and then the light quantity is adjusted by an iris 3 to be exposed on a CCD 5 during a time to open a shutter (not shown). With respect to the light 1, a spatial frequency is restricted by an optical low-pass filter 20 so as to reduce moire and false color, and an area on a long wave side is cut by an IR (infrared radiation) filter 4 before the light is exposed on the CCD 5 so that the CCD does not detect light in an infrared area. The light exposed on the CCD 5 is accumulated as electrical charges to be amplified up to a predetermined gain by a CDS/AGC (Correlated Double Sampling/Automatic Gain Controller) 6, and then converted into digital data by an A/D converter 7. The converted digital image data is subjected to RGB gain adjustment by a white balancing unit 8, and, for example, three color planes are generated by a color interpolation unit 9 as shown in FIG. 2. With respect to the image data of RGB three color planes, adjustment regarding hues of RGB data is performed by a masking processing unit 10, and then a process necessary to display the image data on a display or the like is performed by a gamma converter 11.
If the image data is in a state containing the RGB three planes, there are a large number of data, whereby a compression process such as JPEG (Joint Photographic Experts Group) compression or the like is performed. First, an RGB color space is converted into a YUV color space by an RGB/YUV converter 12, and filtering processes by low-pass filters 14a and 14b are performed to restrict a band for color difference signals U and V. Then, a thinning process to convert a YUV444 format signal into a YUV422 or YUV411 format signal is performed by a thinning unit 15, and the converted signal is finally compressed by a JPEG compression unit 16. Thereafter, the compressed image data is stored in a nonvolatile memory (not shown) or the like in a camera main body.
The conventional digital camera adopting the single plate system rarely has a lens exchangeable system but usually has a lens built-in system. Therefore, an optical low-pass filter, an IR cut filter and the like are previously implanted in front of a CCD to reduce the moire and the false color. On the other hand, with respect to the lens exchangeable system, the IR cut filter can be formed on a glass surface of the CCD as a thin film, but if a space for disposing the low-pass filter is prepared, the camera body substantially becomes large in size. Although the moire or the false color can be reduced by inserting the optical low-pass filter, a spatial frequency of image data decreases in this case. As a result, a sharpened focus as observed in a silver salt camera is lost, whereby importance of an optical system not having the optical low-pass filter comes to be increased.
Even if the optical low-pass filter is equipped, in case of the camera adopting the single plate system of a CCD, since the numbers of R and B pixels are less than the number of G pixel and thus the space between the adjacent pixels is wide as represented by Bayer arrangement, the false color is generated when color interpolation is performed.
As the conventional color interpolation method, a method of forming the three color planes by using a digital filter or the like has been known. However, the number of taps of the filter is restricted in an aspect of hardware, and resolution ability originally held in image data can not be sufficiently extracted.
If a color interpolation process proposed in U.S. Pat. Nos. 5,373,322 and 5,629,734 is performed, it is considered that high-resolution image data can be obtained and an effect to reduce the false color can be attained. However, in the following patterns, the false color is generated.
As shown in FIG. 11, when a vertically striped white line is exposed on lines of G and R components on the CCD with distance of pixel pitch, a vertically striped image of red and yellow is obtained by performing the above color interpolation process. Here, it is assumed that an oblique-line portion indicates black and the data thereof is 0. Similarly, as shown in FIG. 12, when the vertically striped white lines are exposed on G and B lines on the CCD with the distance of pixel pitch, a vertically striped image of blue and cyan is obtained by performing the above color interpolation process. In FIGS. 11 and 12, in case of a horizontally striped line in stead of the vertically striped line, the same result is obtained on the horizontally striped line, not the vertically striped line, whereby horizontally striped images of red and yellow and horizontally striped images of blue and cyan are obtained.
Moreover, as different patterns, when a white image like a checker-flag pattern shown in FIG. 13 is exposed on the R and B lines on the CCD, a complete magenta image is obtained by performing the above color interpolation process.
Similarly, as shown in FIG. 14, when the white image like the checker flag pattern is exposed on the G line on the CCD, a complete green image is obtained in spite of the exposed white image by performing the above color interpolation process.
In order to eliminate the false color, it has been proposed that the color space is converted, for example, from an RGB format into an L*a*b* format and a filtering process is performed respectively to a* and b* components by using the application software on a personal computer.
However, in case of the checker flag patterns as shown in FIGS. 13 and 14, the images can not be distinguished from low frequency images of green or magenta, and the obtained result can not be discriminated as the false color, whereby satisfactory suppression can not be performed.